The subject matter disclosed herein relates generally to nuclide production systems, and more particularly to a nuclide production system that directs a particle beam through a target foil and into a liquid or gas material.
Radionuclides (also sometimes referred to as radioisotopes) have several applications in medical therapy, imaging, and research, as well as other applications that are not medically related. Systems that produce radionuclides typically include a particle accelerator, such as a cyclotron, that accelerates a beam of charged particles (e.g., H− ions) and directs the beam into a target material to generate the isotopes. The cyclotron is a complex system that uses electrical and magnetic fields to accelerate and guide the charged particles along a predetermined orbit within an evacuated acceleration chamber. When the particles reach an outer portion of the orbit, the charged particles form a particle beam that is directed toward a target assembly that holds the target material for isotope production.
The target material, which is typically a liquid, gas, or solid, is contained within a chamber of the target assembly. The target assembly forms a beam passage that receives the particle beam and permits the particle beam to be incident on the target material in the chamber. To contain the target material within the chamber, the beam passage is separated from the chamber by a foil (referred to herein as a “target foil”). A target foil may be a single material composition or two or more layers (e.g. metal sheet coated with another layer). In some cases, multiple discrete sheets may be stacked side-by-side and held together during operation. More specifically, the production chamber may be defined by a void within a target body. The target foil covers the void on one side and a section of the target assembly may cover the opposite side of the void to define the production chamber therebetween. The particle beam passes through the target foil and is incident upon the target material within the production chamber. The target foil experiences an elevated temperature from thermal energy provided by the particle beam.
In many cases, a front foil (sometimes referred to as a “degrader foil” or “vacuum foil”) may be used. The particle beam intersects the front foil prior to intersecting the target foil. The front foil reduces the energy of the particle beam and separates the target assembly from the vacuum of the cyclotron. Although a front foil is frequently used in nuclide production systems, the front foil is not required and a target foil may be used without a front foil.
Target foils for gas and liquid targets also experience an elevated pressure along the side of the target foil that borders the production chamber. Target foils may also experience a corrosive and oxidizing environment due to contact with the target material. The elevated temperatures and pressures cause stress that renders the target foil vulnerable to rupture, melting, or other damage. Target foils may also contaminate the target media when the ions from the target foil are absorbed by the target material.
The most common target foil used today in commercial cyclotrons, especially those that are designed to produce 18F, and in many cases 11C, are Havar® foils. Havar® is an alloy that includes cobalt (42.0 wt %), chromium (19.5 wt %), nickel (12.7 wt %), tungsten (2.7 wt %), molybdenum (2.2 wt %), manganese (1.6 wt %), carbon (0.2 wt %), and iron (balance). Havar® foils have a high tensile strength at elevated temperatures and a thermal conductivity that makes the foils suitable for isotope production. Havar® foils, however, become increasingly radioactive with use, and furthermore, are associated with both chemical and radioactive impurities within the target material. These radioactive impurities can include 96Tc, 51Cr, 58Co, 57Co, 56Co, 52Mn, among others.
Attempts have been made to reduce the amount of impurities within the target material. For example, a niobium (or other refractory metal) layer may be deposited along the surface of the Havar® foil that contacts the target material. Such composite foils, however, can be expensive and may have other drawbacks. Other potential target foils, such as copper, aluminum, or titanium foils, have one or more undesirable qualities that render the foil impractical or less cost-effective for commercial use.